Mantle Convection and Surface Expressions. Группа авторов
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Название: Mantle Convection and Surface Expressions

Автор: Группа авторов

Издательство: John Wiley & Sons Limited

Жанр: Физика

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isbn: 9781119528593

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СКАЧАТЬ velocities of pyrolite appear to systematically fall below those of PREM except for combinations of lowest pressures and temperatures. With magnitudes of less than 3%, these deviations are similar in magnitude to the combined uncertainties on computed P‐wave velocities that arise from propagating uncertainties on finite‐strain parameters, averaging over elastic anisotropy, and mixing mineral compositions with different elastic properties (Figures 3.1 and 3.3a). While affected by the same sources of uncertainties, computed S‐wave velocities appear to be more sensitive to temperature than P‐wave velocities and match S‐wave velocities of PREM, i.e., dlnvS = 0, within the considered temperature interval. The match with PREM would imply a temperature profile that deviates substantially from any of the computed adiabatic compression paths. Modeled S‐wave velocities match those of PREM for temperatures below the central adiabat down to a depth of around 1800 km where temperatures would need to rise above those of the central adiabat in order to follow PREM. Projecting the pyrolitic temperature profile for dlnvS = 0 onto the corresponding map for P‐wave velocities would lead to deviations of –1.5% < dlnvP < 0%. Maps of deviations dlnv for harzburgite are similar to those for pyrolite. However, S‐wave velocities of harzburgite seem to be systematically higher than those of pyrolite by about 0.5% to 1%, and P‐wave velocities show slightly steeper gradients with depths. Computed P-wave and in particular S‐wave velocities for metabasalt are significantly lower than those of PREM at most pressure–temperature combinations explored here. Despite the low volume fraction of free SiO2 phases that go through the phase transition from stishovite to CaCl2‐type SiO2, the related elastic softening of the shear modulus gives rise to a pronounced trough of amplified negative contrasts in P‐ and S‐wave velocities between metabasalt and PREM.

      While the maps shown in Figure 3.8 illustrate the temperature dependence of P‐ and S‐wave velocity deviations from PREM, they heavily rely on assumptions about the Fe3+/∑Fe ratio in bridgmanite and about Fe‐Mg exchange between mineral phases. To explore the impact of these compositional parameters on computed P‐ and S‐wave velocities, I varied the Fe3+/∑Fe ratio of bridgmanite and the Fe‐Mg exchange coefficients between bridgmanite and ferropericlase in pyrolite and harzburgite and between bridgmanite and the CF phase in metabasalt within the ranges suggested by data plotted in Figures 3.6a and 3.6b. When varying one compositional parameter, all other parameters were fixed to their values in the references scenarios. Figure 3.9 shows the resulting deviations of P‐ and S‐wave velocities from PREM assuming a range of Fe3+/∑Fe ratios in bridgmanite or different Fe‐Mg exchange coefficients for each bulk rock composition along the central adiabats shown in Figure 3.8. Computed P‐ and S‐wave velocities of both pyrolite and harzburgite are more sensitive to changes in the Fe‐Mg exchange coefficient between bridgmanite and ferropericlase than to changes in the Fe3+/∑Fe ratio of bridgmanite. The sensitivity to Fe‐Mg exchange increases with depth, in particular for S waves. When combined with a reduction of the Fe‐Mg exchange coefficient images with depth as suggested by recent computational studies (Muir & Brodholt, 2016; Xu et al., 2017), the strong sensitivity of P‐ and S‐wave velocities to Fe‐Mg exchange between bridgmanite and ferropericlase could result in substantial velocity reductions for peridotitic rocks towards the lowermost mantle, even along typical adiabatic temperature profiles.

Graphs depict the relative contrasts between modeled P-wave (upper row) and S-wave (lower row) velocities for pyrolitic (left column), harzburgitic (central column), and basaltic (right column) bulk rock compositions and PREM along adiabatic compression paths starting at 1900 K and 25 GPa. Variations in the Fe3+/Fe ratio of bridgmanite and in the Fe-Mg exchange coefficient have been explored for each bulk rock composition as indicated below the respective diagrams. Dashed curves show P- and S-wave velocity contrasts when the effect of different continuous phase transitions on elastic properties is ignored or modified as specified. Red shaded bands indicate the differences in modeled P- and S-wave velocity contrasts that result from combining the elastic properties of mineral phases and compositions according to either the Voigt or the Reuss bound.

      The Fe3+/∑Fe ratio of bridgmanite dictates the whole‐rock Fe3+/∑Fe ratio and, for pyrolite and harzburgite, appears to affect velocity gradients dv/dz with higher Fe3+/∑Fe ratios leading to steeper gradients dv/dz. Again, the effect seems to be strongest for S waves. Based on a comparison of computed P‐ and S‐wave velocity gradients of pyrolite with PREM, Kurnosov et al. (2017) argued for a reduction of the ferric iron content in bridgmanite with depth. While a steepening of velocity gradients with higher Fe3+/∑Fe ratios of bridgmanite is consistent with the modeling results presented here, an actual match of a pyrolitic bulk composition with PREM seems only possible for S‐wave velocities and at depths in excess of 1500 km. Assuming a Fe‐Mg exchange coefficient of images, modeled S‐wave velocities match those of PREM at about 1600 km depth for Fe3+/∑Fe = 1 in bridgmanite. To maintain dlnvS = 0 at depths greater than 1600 km, the Fe3+/∑Fe ratio of bridgmanite would then need to gradually decrease with increasing depth. For harzburgite, the impact of the Fe3+/∑Fe ratio of bridgmanite on P‐ and S‐wave velocity profiles is complicated by the stabilization of a Fe2O3 component for high Fe3+/Al ratios in bridgmanite due to the lower overall Al2O3 content of harzburgite. As long as sufficient aluminum is available, ferric iron is preferentially incorporated into bridgmanite as the component FeAlO3 (Frost & Langenhorst, 2002; Richmond & Brodholt, 1998; Zhang & Oganov, 2006). While iron cations of the FeSiO3 and FeAlO3 components of bridgmanite replace magnesium on the dodecahedral A site and remain in high‐spin states at pressures of the lower mantle (Catalli et al., 2010; Jackson et al., 2005a; Lin et al., 2016), one Fe3+ cation of the Fe2O3 component is located on the octahedral B site and goes through a spin transition at pressures above 40 GPa (Catalli et al., 2010; Liu et al., 2018). For the modeling results shown in Figure 3.9, Fe3+/Al ratios in bridgmanite become high enough to stabilize the Fe2O3 component only for harzburgite models with Fe3+/∑Fe > 0.5 or СКАЧАТЬ